1. Field of the Invention
The present invention relates to a martensitic steel suitable for transfer steel pipes of oil and natural gas, and having excellent corrosion resistance and weldability.
2. Description of the Related Art
Oil and natural gas supplies have been almost exhausted from easily accessible wells in mild environments, and recent wells must be built in challenging environments, such as severely corrosive environments, cold environments, or in deep wells or in submarine oil fields. Therefore, superior characteristics are required of steel materials used in tubular goods and line pipes in such challenging oil producing regions.
For example, many wells contain large amounts of carbon dioxide gas; carbon steel is highly corroded in such an environment. In order to prevent carbon steel from corroding, inhibitors are injected into pipeline. However, use of inhibitors increases construction and maintenance costs of wells. Further, such inhibitors are not satisfactorily effective at high temperatures. Thus, the recent trend is toward use of corrosion-resistant materials instead of relying on use of inhibitors.
Among corrosion-resistant materials for tubular goods, martensitic stainless steel containing 13 percent by weight (hereinafter referred to as wt %) of Cr is well known. This stainless steel can be produced at low production cost and exhibits excellent corrosion resistance against carbon dioxide gas. However, it is sensitive to sulfide stress corrosion cracking and thus is unsuitable for use in sulfide environments.
Recently, tubular goods of 13% Cr steel containing Mo, Ni and the like have been developed as disclosed in, for example, Japanese Unexamined Patent Publication No. 60-174,859 in order to adapt to environments containing small amounts of hydrogen sulfide. These tubular goods exhibit excellent resistance to sulfide stress corrosion cracking (SSC resistance).
On the other hand, the API Standard defines 12% Cr martensitic stainless steel containing reduced carbon as a line pipe material. However, this stainless steel requires preheating and postheating during circumferential welding, resulting in increased cost and toughness deterioration of the weld section. Thus it is little used.
Because 13% Cr steel containing Ni and Mo does not take into consideration weldability, weld cracks will surely occur when welding line pipes made of such a steel without preheating and postheating.
Accordingly, dual-phase stainless steel, which exhibits excellent weldability and corrosion resistance, has been used in line pipe materials. However, dual-phase stainless steel has unnecessary high cost for some oil and natural gas wells, causing increased well construction costs.
When hot oil or gas flows in a line pipe, the strength of the line pipe decreases. Thus, various corrective measures have been taken, for example, increasing room temperature strength of the line pipe, and increasing pipe thickness. However, increasing room temperature strength has been found to deteriorate weldability, and increasing the pipe thickness of course increases material and production costs.
In summary, no conventional line pipe material exhibits adequately stable corrosion resistance, satisfactory toughness of the weld heat-affected zone, and satisfactory high temperature strength. Further, all the known materials have serious cost disadvantages.